XLR out without isolation-transformer

[Chris / CMW amps]

Hi all

Is it possible to install a(n) XLR-out on an amp without using an isolation/balanced-transformer?

I'm wondering about using a resistor on the "-"-side of that output...

Many tia!

[Albert Kreuzer]

Hi Chris,

from the preamp you could make up something like this:
http://www.albertkreuzer.com/pics/el...hem/di_out.gif

Also possible with tubes (voltage dividers and cathode followers after the PI?).

In a non-NFB circuit you could disconnect the ground connection of the OT secondary and do a virtual center tap (like the heater circuit, but with two voltage dividers and blocking caps for the outputs).

Just an idea, did not build this yet.

Cheers,
Albert
www.albertkreuzer.com

[Enzo]

It is done all the time in commercial amps, and pretty much in the way Albert has done it. Drive one pin and invert the signal for the other. I see darn few balanced outs using transformers.

Or do you require it to be all tube?

[Chris / CMW amps]

Hi Albert and Enzo

Thanks for your suggestions! I'll just see what the amp and customer want.

Again thanks and have a very nice weekend!

[Mark Black]

Just one addition:

Although not shown in the schematic be sure to use at least 63V (or preferably 100V) rated caps in the 22uF output section to protect against possible 48V phantom power from a console. Also I would think the polarity of those caps should be reversed for the same reason (positive toward XLR).

[Guitarist]

I love the idea of tapping off the OT and having an ungrounded speaker jack.
I asked this a few years ago with the notion to send out from the PI caps which are "balanced" as well but IIRC it would've been too large AC dB reading. If it was padded down with ??Ohm resistors it should work, eh?

If this could fly what value of R? There may be a bit of phase shift but that's a concern with alot of cheaper tube mics as well that are transformer coupled e.g. Carvin, Nady, etc.

Also, with a feedback loop it might be possible too and good for the masses. Isolation would be needed I suppose but such is beyond me.